In a cellular communication system using spread spectrum technology, such as Code Division Multiple Access (CDMA), the spectrum is divided into 40 frequency bands. The 40 bands are divided between wireline and non-wireline applications, generally assigning 20 bands to each. The 20 bands are equally divided with 10 bands used for forward communication with the base station as the transmitter and 10 bands used for reverse communication with the mobile as the transmitter. Each band is typically 1.2288 MHz wide and will handle multiple users simultaneously.
In this description, calls and users are used interchangeably. A band that supports X users supports X calls. The capacity of a band in number of users always refers to full rate (9600 baud) users unless otherwise stated. A band that supports X full rate (9600 baud) users, will support 2X half rate (4800 baud) users, or 4X quarter rate (2400 baud) users. The band will also support a certain number of variable rate users (9600, 4800, 2400, or 1200) with the exact number of users dependent on the amount of time spent at each baud rate.
In operating at system capacity, accurate mobile transmit power control is very important. Mobile power is controlled from the base station by transmitting power control data to every transmitting mobile. The base station can direct each mobile to increase or decrease its transmitting power, typically in preset increments (e.g. 0.5 dB).
Channel interference is an on going problem in any communication system. The interference is generally caused by a combination of thermal noise (KTB), man made noise, and co-channel noise. Co-channel noise results from other calls on the same channel, either within the same cell or from another cell. As the interference within a cell increases, the base site will direct the subscriber units to increase power. This increased power will cause interference in other cell sites which will then raise their power to compensate. This will escalate until the maximum power output of one or more subscribers is reached. At this point, the signal-to-noise ratio will drop below a preselected minimum level and one or more calls will be dropped.
Therefore, it is an on-going effort for cellular designers in a CDMA system to maximize the number of users on a system while at the same time avoiding the noise problems caused by those users.